A nanosheet obtained by the exfoliation of an inorganic layered compound is known to be a substance having as a high two dimensional anisotropicity as several hundreds nm to several tens μm in lateral size relative to a thickness of several atoms. For example, among the inorganic layered compounds, a layered oxide is reported to exhibit a unique property reflecting the structure of the nanosheet mentioned above including a semiconductor property, a dielectric property, a ferromagnetic property, a fluorescent property, an optical catalytic property and the like.
In order to allow these properties to be exerted sufficiently, it is required to accomplish a formation of a highly defined nanosheet monolayer film in which the nanosheets are regularly aligned with an accuracy at a nanometer level while suppressing overlap or gap formation, and to repeat it to construct a multilayer film.
A technology for the aforementioned purpose includes a method involving a combination of an alternative adsorption method and an ultrasonic treatment method (see for example, Non-Patent literature 1). In Non-Patent literature 1, a substrate pre-coated with a polydiallyldimethyl ammonium (PDDA) ion having a positive charge is immersed in a colloid suspension having a negative charge for 20 minutes to allow the nanosheet to be adsorbed onto the substrate as a result of an electrostatic attractive force, followed by an ultrasonic treatment to remove the overlapped portions, thereby obtaining a satisfactorily aligned nanosheet monolayer film.
Another known technology is Langmuir-Blodgett (LB) method (see for example, Non-Patent literature 2). In Non-Patent Literature 2, on a trough where a colloid suspension is spread, a surface pressure is applied to allow the nanosheet to be adsorbed onto a gas-liquid interface, and then to be transferred onto a substrate, thereby obtaining a satisfactorily aligned nanosheet monolayer film.
Nevertheless, any of the technologies of Non-Patent literatures 1 and 2 involves a prolonged film-forming time (for example 2 hours in Non-Patent literature 1 and 1 hour in Non-Patent literature 2), a complicated film-forming operation or a reduced surface area of the substrate capable of film-forming, because of which a simple and highly productive novel film-forming technology is desired.